The present invention relates to a new grain-oriented magnetic steel sheet with an electrically insulating coating which is applied after final annealing in order to ensure electrical insulation of the individual layers of sheet, for use of the grain-oriented magnetic steel sheet e.g. in transformers. The invention also relates to a method for producing the grain-oriented magnetic steel sheet with an electrically insulating coating.
For further use, e.g. in transformers, it is important to reduce the hysteresis loss. One measure for this which is frequently used is to add by alloying silicon which results in an increase in the specific electrical resistance and thus in a reduction in eddy-current losses. By means of modifications in the chemical composition and the cold-rolling and annealing processes, crystal orientation {110} <001> is set and enhanced. By means of a reduction in the thickness of the sheet, the loss is further improved. Moreover, by improving the purity of the steel, it is possible to avoid precipitated particles in the finished product which as undesirable traps impair the Bloch wall movement during magnetic reversal.
Types of magnetic steel sheet with particularly enhanced orientation and thus high permeability can be still further improved regarding the hysteresis in that the production process is controlled such that limitation of the size of the secondary recrystallised grains, and, respectively a large ratio of grain boundary length to grain surface, is ensured, and thus the Bloch wall spacing is reduced. The state of the art also includes additional improvement of the domain structure by way of applying an insulation layer which exerts a permanent tensile stress on the sheet substrate, and additionally by treatments which generate lines of local tension across or inclined to the direction of rolling. Among other things these can be local mechanical deformations (EP 0 409 389 A2), laser beam or electron treatments (EP 0 008 385 B1; EP 0 100 638 B1; EP 0 571 705 A2) or the etching-in of grooves (EP 0 539 236 B1).
This method of producing types of magnetic steel sheets with particular low-loss characteristics is associated with a disadvantage in that the combination of measures for forming the insulating layer and further domain refining is expensive. There is a further disadvantage in that the insulation layer is usually constructed in a series of complicated process steps which are carefully attuned to each other. This provides very little scope for undertaking still further parameter variations for economical and qualitative process optimisation.
The hitherto commonly used tension-applying layer is implemented in that the strip which has been cold-rolled to final thickness is subjected to annealing for primary recrystallisation and decarburising, wherein in a targeted way the surface is oxidised, then coated with MgO and suitable additives as a non-stick layer, and dried, and subsequently coiled and again annealed for the purpose of secondary recrystallisation and subsequent cleaning of the steel of precipitation-forming elements. During this annealing step, the non-stick layer reacts with the oxides on the strip surface and forms a forsterite layer (Mg2SiO4) which is also referred to as a “glass film”. This film becomes rooted in the base material, a characteristic which enhances its adhesion.
In a further process step, as is for example known from DE 22 47 269 C3, solutions based on magnesium phosphate or aluminium phosphate or mixtures of the two with various additives such as for example chromium compounds and Si-oxide are applied to said film and burned-in at temperatures above 350° C. The tensile stress which the finished insulation layer transfers to the base material can be up to approx. 5 MPa. The improvements in hysteresis loss achieved in this way are of a magnitude of approx. 5%. Furthermore, magnetostriction is reduced.
The achievable improvement in loss is limited by the fact that, for forming the layer, oxidation processes are inevitable during which non-ferromagnetic particles and inhomogeneities form at the surface or in the surface zones, with said particles and inhomogeneities impeding the mobility of the Bloch walls during magnetic reversal, thus causing increased energy losses.
In newer developments, attempts have therefore been made to produce magnetic steel sheet without a glass film and with a surface which is as smooth as possible; and afterwards to apply tension applying insulation layers which do not require surface oxidation as a base. For example, sol-gel methods for layers with oxidic substances have been tried, as described in EP 0 555 867 A2. In this arrangement, the layer tensions have been created on the basis of the difference in the thermal expansion coefficients of the steel and the layer, and on the basis of the high temperature of between 800° C. and 1000° C. during formation of the layer. Other known methods include the application of thin layers onto sheet substrates made of magnetic steel sheet of extremely smooth surface by means of CVD or PVD methods such as electron beam evaporation, magnetron sputtering or vacuum arc evaporation, wherein layers or multiple layers of metal nitrides or metal carbides (e.g. TiN, BN, ZrN, AlN, Ti(CN), Cr2N, TiC, ZrC, WC) are produced, as described in EP 0 193 324 B1 or EP 0 910 101 A1.
With these types of layers it is possible to create tensile stress in the magnetic steel sheet of for example 8 MPa, however, their inadequate electrical insulation effect is disadvantageous so that they have to be covered by an additional insulating layer as described in EP 0 215 134 B1.